Figure 3 expounds how to achieve both high speed and low power. By the reduction of switching current, lower power becomes available because of the relationship in equation 1. The most important factor for satisfying both high-speed and low current is to use small-size p-MTJs with small damping constant. If sufficiently small activation area with small damping constant and high retention energy were satisfied, the reduction of the switching current would be available in proportion to the reduction of the area of storage layer when activation area is smaller than cell area. The switching currents are also proportional to the damping constants. However, according to previous reports, the damping constants in perpendicular magnetic materials had been too big (more than 0.01). Therefore, developing the small damping constant has been essential for achieving both fast speed and low switching current. In this study, the p-MTJ film with smaller damping constant (0.004) consisting of top electrode/reference layer/MgO/storage layer/bottom electrode has been used as p-MTJ cells.
Figure 3: Methods to satisfy both fast and low-current writing. By adopting the smallest damping constant of 0.004 and p-MTJ cell miniaturized to 30 nm, high-speed of 3 ns and low-current 50 μA spin transfer torque writing has been enabled.
By using damping constant of 0.004 and miniaturization to 30 nm, high-speed of 3 ns and low-current write of 50 μA were obtained. The damping constant of the storage layer with intrinsic perpendicular magnetic anisotropy Ku=1.4 × 107
is measured by a ferromagnetic resonance (FMR) method. The Ku is estimated from figure 4. The high Ku can be expected to improve retention energy.
Figure 4: The hysteresis curve (M-H loop) for storage layer. Blue line and red line indicate a magnetic field (Oe) applied in a direction perpendicular and in-plane to a film surface, respectively.
The damping constant is obtained from a spectrum of the microwave absorption result shown in figure 5. Figure 6 shows the applied magnetic field angle dependence of damping constant. The minimum damping constant of 0.004 has been obtained. From the above results, the simultaneous achievement of low damping constant and high retention energy can be expected.
Figure 5: FMR differential absorption spectrum of a storage layer. The damping constant was estimated form the peak–to-peak width. The peak-to-peak width is 28 Oe in magnetic field perpendicular to the film plane.
Figure 6: Applied magnetic field angle dependence of damping constant. Damping constants are estimated from ΔH obtained in Figure 5. Minimum damping constant is 0.004 at perpendicular magnetic direction.
Figure 7 shows a cross-sectional TEM image of the 30 nm p-MTJ cell. A circular p-MTJ cell shape was selected to reduce the cell area.
Figure 7: Cross-sectional TEM image of 30-nm p-MTJ. There is a storage layer between two white lines. Upper white line is tunnel barrier layer, and there is a reference layer on tunnel barrier layer.
Performances of a p-STT-MRAM cell and NVMRAM
Figure 8 shows repeatability of writing and reading. 600 mV 3 ns writing pulse (a) and 240 mV 3 ns reading pulse (c) are applied in the 30 nm p-MTJ cell. Good writing and reading repeatability without miss-writing was confirmed in 100 to 200 time trials. Since writing currents of 48 μA and 38 μA are used, PE is estimated to be less than 0.09 pJ from the above results. Figure 9 shows the time dependence of average switching current. In order to prevent miss-writing, reading currents less than Ic and writing currents larger than Ic are needed.
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Figure 8: (a) Repeatability of spin transfer torque writing. Writing pulse selected is 3 ns/±600 mV. (b) is measurement procedure of (a). Repeatability of reading is shown in (c). Reading pulse selected is 3 ns/±240 mV. There is no write error due to read-out operation. During each measurement, external magnetic field of 1338 Oe is applied for perpendicular direction to cancel the stray field from reference layer.
Figure 9: Pulse width as a function of switching current. White points and black points show switching currents from parallel state to antiparallel state and from antiparallel state to parallel state, respectively. Star mark indicates the writing conditions and reading conditions for Figure 8.